Pediatric anesthesia texts currently recommend limiting narcotic analgesics to infants over 6 months of age because younger infants are suscepted to have greater sensitivity to narcotic- induced respiratory depression. In order to determine the time course of respiratory depression following narcotics in infants, to determine its mechanisms and the time course of maturation of this sensitivity, we will study the pharmacokinetics and ventilatory effects of morphine, and fentanyl in infants from 1 day to 2 years of age. Parallel studies in macaques (premature to adult) will relate ventilatory effects of morphine (and fentanyl) to serum and cerebrospinal fluid levels of narcotic to assess the role of age-related differences in CSF distribution and brain uptake of narcotics. Age-related differences in narcotic metabolism and their mechanism will be studied with the intent of identifying infants at risk for prolonged narcotic effects. The results of these studies will allow safer use of narcotic analgesics in infants undergoing procedures where these medications are needed in adults.